1. Field of the Invention
The present invention relates generally to a process for fabricating a membrane structure and more particularly to a process for fabricating a membrane structure which is especially adapted for use as a mask substrate for X-ray lithography in which a fine pattern is transferred and exposed by using soft X-rays to form a pattern for the manufacture of electronic devices such as integrated semiconductor devices.
2. Description of the Prior Art
In a conventional process for fabricating a membrane structure, a first thin film which becomes a membrane is first formed over one major surface of a substrate. Simultaneous with or after the formation of the thin film over one major surface of the substrate, a second thin film is also formed over the other major surface of the substrate. Thereafter, the second thin film is removed by a suitable treatment like photo-etching except a portion which remains as a frame. Next, the substrate is removed except a portion, which remains as a frame, by using an etchant which etches only the substrate, whereby a membrane structure is obtained.
Such membrane structure is used as a substrate for fabricating a mask for X-ray lithography. In this case, not only must the membrane have a high degree of transparency to X-ray but it must also be sufficiently transparent over the whole wavelength range of visible light in order to use effectively an optical alignment apparatus which is used to align patterns with a high degree of precision when the patterns are transferred by using X-rays. The most fundamental requirement for membrane structure resides in the fact that a membrane is subjected to tension with a suitable tensile stress, so that it is prevented from sagging. Therefore, it is preferable that the membrane has a tensile stress from 0.5.times.10.sup.9 dyne/cm.sup.2 to 3.times.10.sup.9 dyne/cm.sup.2.
In order to fabricate a membrane consisting of an inorganic material especially such as silicon nitride (Si.sub.3 N.sub.4), the Chemical Vapor Deposition (CVD) process has widely been used in which a gas such as SiH.sub.2 Cl.sub.2 or NH.sub.3 is used as a raw material and the thermal reaction between SiH.sub.2 Cl.sub.2 and NH.sub.3 is utilized at a high temperature (700.degree.-900.degree. C.). When the CVD process is employed so as to form a Si.sub.3 N.sub.4 film which becomes a membrane, the film generally has a high tensile stress (of the order of 10.sup.10 dyne/cm.sup.2). Therefore, M. Sekimoto et al. proposed that a thin film of silicon rich SiNx (x&lt;4/3) be formed by controlling the reaction temperature and the composition of gases so that the tensile stress may be reduced (See "Silicon nitride single-layer X-ray mask", J. Vac. Sic. Technol. 21(4), Nov./Dec. 1982, pp. 1017-1021). But, this process has a defect that transparency to visible light, especially light on the short wavelength side, is degraded considerably. Furthermore, it has disadvantages in that the stress cannot be controlled optimally and a high yield cannot be ensured.